In the manufacture of coated paper, to obtain paper having a smoother and microporous surface, one or more coatings serving to fill in the roughnesses in the paper surface and thereby to equalize that surface are applied to the uncoated paper. Suitably, an excess of wet coating composition is applied to the paper web by means of a premetering device.
In order to provide a uniform thickness of the coating on the paper web, a doctor, the so-called doctor blade, extending across the width of the paper web is arranged at some distance from the premetering device.
It has been known for many years that at high speeds of a paper web, which high speeds are desired to realize the highest possible production, problems arise in the area downstream of the doctor blade, seen in the direction of travel of the paper web, (the so-called "dry" side of the doctor blade) owing to the fact that in that area, across the length of the doctor blade, a liquid or hard edge is formed consisting of coating composition which may or may not be mixed with fibres. In the technical literature the formation of such an edge is called "stalagmite formation" or also "bleeding", "whiskering" or "feathering". In the present text the term stalagmite formation will be used.
It is known from the literature that the stalagmite formation assumes more serious proportions as the speed of the paper web increases. At the moment speeds of 1,000 to 1,500 m/min are already customary and for the future even higher speeds are not impossible. Moreover, the gravity of stalagmite formation depends on the composition of the coating, for which every paper maker has his own specific recipe, which coating, however, will always contain a pigment constituent, such as clay or chalk, and a binder, such as a latex or a modified starch. It is desirable both for the quality of the paper and for the production costs thereof to apply a coating having a high solids content. Such a high solids content, however, is also found to promote a strong stalagmite formation at the doctor blade.
The formation of a stalagmite on the "dry" side of the doctor blade is highly undesirable, because pieces of coating composition may get loose from this stalagmite, especially when it assumes too large sizes, which pieces, when arriving at the paper web, may damage the paper surface, as they cause streaks and cavities. Furthermore, hard pieces of stalagmite may damage the surface of the rolls over which, in the process of papermaking, the paper web passes after application of the coating. In particular the surface of the so-called calender rolls to be used for the further smoothing of the paper surface is susceptible to this. Replacement of such rolls is very expensive. Finally, in the subsequent process of printing paper hard pieces of coating composition which are still present in the paper after the manufacturing process may lead to problems or to disfigured printed matter. In addition, the occurrence of stalagmite restricts the selection of the raw materials to be used in the coating composition, which may adversely affect the quality of the paper and/or the costs of raw materials.
For a more detailed description of the problem described above, reference can be made to three articles in the "Wochenblatt fur Papierfabrikation", namely "Streichfarben mit hohem Feststoffgehalt--Rheologie und Verarbeitbarkeit beim Bladestreichen" by G. Engstrom in Vol. 6, 1984, pages 184-187; "Fabrikationsstorungen durch stalagmitenformigen Streichfarbenaufbau an den Schabern schnell laufender Streichmaschinen" By H. P. Hofmann and A. von Raven in Vol. 8, 1986, pages 261-265; and "Betrachtungen uber Stalagmitenbildung und Bladeuberkochen" by D. Eklund and S. Fors in Vol. 10, 1988, pages 400-404. Two articles published in "Tappi" in 1973 shows that the disadvantageous effects of stalagmite formation have been recognized for may years. These articles are: "Effect of coating color rheology on the blade coating process" by J. P. Modrak in Vol. 56, No. 10, October 1973, pages 70-73 and "Effect of hydrocoloids on coating color operability and coating paper properties" by E. J. Barbar in Vol. 56, No. 10, January 1973, pages 52-55.
It will be clear from the above that in the paper coating operation it is highly important to prevent stalagmite formation without concessions being required with regard to the desired high speed of the paper web and/or the makeup of the coating composition and/or the properties of the paper to be coated and more in particular with concessions being required with regard to the desired relatively high solids content of the coating composition.